1. Field of the Invention
This invention relates to an output circuit used in a semiconductor device with many pins, such as a memory device or an arithmetic unit, and more particularly to a high-speed output circuit.
2. Description of the Related Art
Output circuits of this type are required not only to have a high bus driving capability for high speed operation, but also to suppress noises arising from the simultaneous operation of many output circuits.
FIG. 1 is a circuit diagram of a conventional output circuit. The output circuit is composed of a p-channel MOS transistor 11 for producing a high-level signal and an n-channel MOS transistor 12 for producing a low-level signal. Specifically, the gate of MOS transistor 11 is supplied with a first control signal S1. The MOS transistor 11 has its source connected to the power supply V.sub.DD, and its drain connected to both the output terminal 13 and the drain of MOS transistor 12. The MOS transistor 12, whose source is connected to the ground, has its gate supplied with a second control signal S2.
The current driving capability of the conventional output circuit is dependent on the power supply voltage and temperature. The impedance of the signal transmission system is not power voltage- or temperature-dependent, but is constant. An ON-resistance is set to the MOS transistor of the output circuit so as to be able to match with the impedance of the signal transmission system when the power supply voltage and temperature are at standard levels. Under conditions of high power supply voltages or low temperatures, the current driving capability of the output circuit increases to lower the ON resistance of the MOS transistor. As a result, the ON resistance cannot match with the impedance of the signal output system, increasing the number of occurrence of noise. Under conditions of low power supply voltages or high temperatures, however, the current driving capability decreases seriously. Thus, it is difficult to reconcile a high bus driving capability with low noise.
To meet both incompatible requirements, a high bus driving capability and low noise, the conventional output circuit is designed to have such a current driving capability as avoids the faulty operation under high-driving conditions liable to a lot of noises, that is, at high power supply voltages or low temperatures. With this design, however, there arises a problem that it operates slower under low-driving conditions, that is, at low power supply voltages or high temperatures.